As the speed of wireless data communication increases, more communication performed via land lines is being replaced by wireless communication. However, electric power is supplied by land lines, and wiring needs to be installed to supply power to wireless communication devices. In order to advance the wireless revolution to include both communication and the supply of power, technologies for supplying electric power wirelessly need to be developed.
Non-contact power transmission techniques have gained attention as a way to supply electric power wirelessly (see, for example, Japanese Laid-open Patent Publication No. 2009-261156: Patent Document 1), but these techniques supply power using non-emitted energy based primarily on electromagnetic induction. As a result, the power transmission efficiency declines as the distance between the power transmitter and the power receiver increases, and the practical range for this technique extends from several millimeters to several dozen centimeters. Even in the case of the electromagnetic field resonance method, the supply of electric power is limited to short distances of one meter or less. These non-contact techniques are effective at relatively short-distance power supply, but they are no substitute for both indoor and outdoor wiring.
Power transmission techniques via microwaves and laser beams in free space have been researched as another type of non-contact power transmission method. This research has focused primarily on long-distance power transmission techniques between the surface of the earth and outer space, but it will take time to develop practical uses due to problems unique to long-distance power transmission techniques.
Laser power transmission has recently been proposed as a power transmission method for moving objects such as electric vehicles. For example, a laser beam power transmission system has been disclosed in Laid-open Patent Publication No. 2010-166675 (Patent Document 2) which comprises an electric vehicle which receives a laser beam, converts the laser beam to electric power and uses the resulting power to drive the vehicle, and an electric power supply installed outside of the electric vehicle for supplying the laser beam to the electric vehicle while automatically adjusting for the relative position between the vehicle and the power supply. The laser power transmission technique in Patent Document 2 is a relatively short-distance technique in which a laser beam is emitted from a phase array-type light-emitting device installed in the overhead structure of a bridge or tunnel to a phase array-type light-receiving device installed in the roof of an electric vehicle. These laser power transmission techniques have not been developed for practical use because of problems such as alignment difficulties and inefficiency.
The International Electrotechnical Commission has issued “Safety of Laser Products—Part 1: Equipment classification, requirements, and user's guide” IEC 60825-1, regarding the safety of laser devices. The international safety standards, and the safety standards of countries that comply with the international standards, regulate laser devices based on indicators known as maximum permissible exposure (MPE) and accessible emission limit (AEL). Laser devices are classified according to these safety standards, and some laser products may be assigned to a class lower than that of the actual power of the laser through technical means employed to limit exposure, such as housings and safety interlocks. DVD devices, Blu-ray (registered trademark) devices and laser printers are products that are marketed in accordance with such safety standards.
Laser devices with output in the 1 W range are also used in laser displays and laser light shows in concert halls. Laser devices which emit a scanning laser beam are classified according to the emission of the scanning laser beam. As a result of scanning failures, such as changes in scanning speed or scanning amplitude, safety precautions have been taken so that exposure exceeding the accessible emission limit (AEL) for a given class does not occur.
A power transmission technique using the emission of electromagnetic waves such as laser beams is desired in order to create truly wireless communication devices able to communicate data at high speeds and receive power wirelessly. However, the emission output has to be increased in order to supply power adequately. Therefore, the development of a technique is desired which can transmit power via the emission of electromagnetic waves at even higher outputs while also meeting exposure standards.
An emergency stop function for the supply of power via light has been disclosed in Laid-open Patent Publication No. 11-230856 (Patent Document 3). In Patent Document 3, a configuration is disclosed in which the presence of optical feedback anomalies is detected via a separate optical fiber housed in the same cable as the optic fiber transmitting the optical power. However, the technique in Patent Document 3 relates to the supply of optical power via wiring, and does not contribute to advancing the wireless revolution to include the supply of electric power.